This invention relates generally to a power measuring apparatus, and more particularly to a calorimeter type apparatus for measuring radio wave power of a millimeter-wave or the like.
Heretofore, for the measurement of millimeter-wave power in the range of several microwatts to a hundred milliwatts, a system called bolometer measurement is frequently employed, as in the case of a microwave band, in which a bridge and a bolometer mount are used in combination and a thermistor or barretter is used as a power absorbing element. In the millimeter-wave power measurement, however, extreme deterioration is unavoidable in the substitution efficiency of the bolometer mount due to a remarkably high rate of a thermal loss caused principally by the wall of a waveguide. For this reason, the power obtained through substitutional measurement by the bolometer mount becomes considerably different from the true power, and this brings about the necessity of calibrating the substitution efficiency of the bolometer mount by some proper means to ensure precision measurement of millimeter-wave power.
Presently, in measurement of a small-value power of a millimeter-wave band, the highest accuracy is achievable by the use of a calorimeter. Among the apparatus developed for this purpose, a microcalorimeter is available for measuring the effective efficiency of the bolometer mount. Here, the term "effective efficiency" is defined as a substitution efficiency representing the ratio of the power measured by the bolometer to the true power absorbed by the bolometer mount. The principle of this system is illustrated in FIG. 1.
FIG. 1 is a diagram showing the operating principle of a conventional microcalorimeter for the measurement of a millimeter-wave power, in which a bolometer mount 1 is used as a common thermal load for a millimeter-wave power and a DC power for substitutional measurement. A thermoelectric detector element 5 senses a temperature rise occuring in the bolometer mount 1 as a result of absorbing both the DC power fed to a bolometer element 3 from a bolometer bridge 2 and the millimeter-wave power fed from a waveguide 4. Through negative feedback of the output of the element 5 to a thermoelectric cooler element 7 via a cooling current controller 6, the bolometer mount 1 and a reference temperature jacket 8 are controlled isothermally.
Millimeter-wave power is applied under such control to perform substitutional power measurement by the bolometer mount 1 and the bolometer bridge 2, and simultaneously a cooling current is obtained by measuring the voltage across a standard resistor 9 by means of a voltmeter 10. The effective efficiency of the bolometer mount 1 can be determined from the results of such two measurements. The bolometer mount 1 with the thus determined effective efficiency may be used as a standard for calibrating the effective efficiency of other bolometer mounts through comparative measurement. This will now be further described in detail. In the conventional measurement system shown in FIG. 1, input power is expressed as follows: EQU P=.alpha.(.theta.+273.degree.) I.sub.f -1/2r I.sub.f.sup.2 +(.theta.-.theta..sub.j)g+C d.theta./dt (1)
where
P: input power (W) to thermal load PA1 I.sub.f : Peltier cooling current (A) PA1 .theta.: thermal load temperature (.degree.C.) PA1 .theta..sub.j : temperature (.degree.C.) of reference temperature jacket PA1 g: thermal resistance (W/.degree.C.) between thermal load and jacket PA1 .alpha.: thermoelectric conversion coefficient PA1 r: electricl resistance (.OMEGA.) of Peltier element PA1 C: thermal capacity of thermal load
Supposing now that the conditions are established as EQU .theta.=.theta..sub.j =.theta..sub.o (constant) and EQU .alpha.(.theta.+273.degree.) I.sub.f &gt;&gt;1/2r I.sub.f.sup.2
then Equation (1) can be written as follows: EQU P=.alpha.(.theta..sub.o +273.degree.) I.sub.f =KI.sub.f ( 2)
Thus, it denotes that the input power to the thermal load can be obtained from the Peltier coefficient K and the cooling current I.sub.f. However, since the Peltier coefficient K is a function of the temperature .theta..sub.j of the reference temperature jacket 8, it is necessary to suppress the change of .theta..sub.j within, for example, .+-.2/10,000.degree. C. to attain high-accuracy measurement.
For the above reasons, the use of an excellent thermostatic oven is essential. This requires a bulky construction of the apparatus, hence inducing disadvantages in practical use including that the preparation time required for stabilizing the entire apparatus is extended to be as long as several hours.
Furthermore, there are difficulties in producing satisfactory bolometer mounts having both superior matching characteristics and substitution efficiency for use in the millimeter-wave band, and this causes a decrease in the power measuring accuracy.
Accordingly, in precision measurement of a millimeter-wave power, there exists possibility of attaining a higher accuracy by absorbing, without using any bolometer mount, the millimeter-wave power into a small matching load easily producible and then executing direct measurement of the power calorimetrically.
In such calorimetric measurement, the object to be measured is a power or a ratio of two compared powers. Accordingly, the highest accuracy is attainable by the method of measuring the true incident power directly from the known substituted power, without measuring the cooling current as in the aforementioned. That is, since the thermoelectric cooler element 7 used in the microcalorimetric measurement is composed of semiconductor, there exist some disadvantages including that an error resulting from non-linearity of the cooling current or drift of the ambient temperature is basically unavoidable and, in addition to complicated control action, a long measuring time is required due to the large time constant as the feedback control is carried out by the thermoelectric cooler element 7 of a great thermal capacity.